Bandgap is one of the characteristic properties of semiconductors. It represents the energy gap between electronic orbitals (or more rigorously speaking, electronic bands) filled with electrons (the valence bands) and the ones with no electrons (the conduction bands). The bandgap largely determines the optical and electrical properties of a semiconductor.
Though the bandgap is a semiconductor's intrinsic property, its value does not always remain constant. Quantum mechanics reveal that, when the size of a semiconducting material falls in the nanometer range, its bandgap starts to broaden. This behavior is due to the increased quantum confinement effect imposed by size reduction, which shrinks the available space for electrons (sometimes are positively charged holes) in semiconductors to move around. This increasingly confined space leads to the enlarged bandgap, as predicted by Schrödinger Equation (a fundamental equation that every student must master in order to survive a Quantum Mechanics 101 class).
However, my knowledge on quantum mechanics has been completely thrown up after I came across a paper recently published in The Journal of Physical Chemistry C 2018, 122, 9292-9301. Sharma et al. reported an anomalous trend of how the bandgap of hematite (the alpha phase-iron oxide), a semiconducting material, changes with crystal size. The bandgap of hematite indeed increased when the crystal size reduced from 75 nm to 30 nm. However, when further decreasing the size from 30 nm to 15 nm, the bandgap decreased (see the figure below, the blue curve). How come what I learned is not true here?
Figure. Variation of electrical conductivity (the black curve) and bandgap (the blue curve) with crystallite size of hematite. Credit: American Chemical Society.
It turned out that what I learned touches only the basic but the real world is always more complicated. The authors of the article studied the evolution of the electronic structure of different sized hematite nanoparticles using X-ray absorption spectroscopy. It reveals that the observed "abnormal" observation, in a nutshell, is because of the change in bond nature between Fe and O atoms. This bond change alters the width of the valence band and consequently modifies the bandgap of hematite. My quantum mechanic class assumes the width of the valence band is constant and, hence, the conclusion derived there is no longer valid for this case.
This experience reminds me about the difference between textbook knowledge and cutting-edge research: Textbooks help us to form general yet somewhat blurred images, but it is the insight gained from state-of-the-art experimental works that render pictures with much improved resolution. It is why I feel extremely excited when reading cutting-edge research works both within and beyond my research expertise.
Comments